Transmitting real-time traffic on full-duplex lines over packet networks is expected to play an important role in future telecommunications. Digital based devices are superseding their classical analogue counterparts in public switched network, increasing the demand for analogue-digital (A/D) conversion, data coding/decoding, echo cancellation and compression/decompression.
The devices involved are typically shared by several sources using time division multiplexing (TDM). Time domain multiplexing of data streams of several source requires exact synchronization of those sources to be performed efficiently.
In a full-duplex voice and/or video communication, need for synchronization is pronounced by having in almost every case a local source and a remote source which communicate. The local source defined as being physically close the data signal processing apparatus provides a signal with low latency or jitter and a constant bit-rate. The signal of the remote source while crossing the network is subject of end to end delay, and comparably large jitter. Depending on the quality of the intermediate network, connection parameters like cell delay variation or burst size can only be approximately guaranteed for the remote source.
An asynchronous process consists in receiving and processing the packets when they arrive in the input packet stream. This method is not a problem when the processor is not loaded. However, when signal processing requires to process the signal transmitted from the remote source and from the local source within the same time slot, synchronization measures to secure this simultaneous process are necessary. The synchronous process may consist in using a timer for re-synchronization: this method requires the access to an external clock such as the network clock. Usage of a timer is applied in the system described in the European Patent EP224626. In an telecommunications access node the port adapters, thanks to the network lines attachments, can re-synchronize the traffic using the clock of the network. However, in a processing server which is a part of the network node with no external network line attachment, it is not simple to retrieve the clock from another part of the node.
In view of the prior art, it is therefore an object of the current invention to provide a method and apparatus for synchronizing a remote and a local source of traffic to be Processed as to minimize overrun and underrun of the processor when there is a difference in the frequency of both sources without using an external clock. As many connections are established on the same lines, it is a further object of the invention to minimize delays when feeding signals of several pairs of local and remote sources (connections) to the shared signal processing device.